you would need to change /dev/sda2 to /dev/mmcblk1p1 as that is the first partition on the SD card as detected by the kernel. Also, the pinebook will try to boot on the MicroSD if installed. you could install i during boot but you might not install it in time to have it detect it and mount it. The nofail setting means it won't crash without the card, but unless you insert the card between start of boot and detection time (this is probably a very small window) you will want to open the MATE terminal and run sudo mount -a to mount the drive after boot.